Turbochargers for internal combustion engines have been widely used on both diesel and gasoline engines for many years. A great deal of effort was expended in the early years of turbocharger development to produce a bearing system that exhibited sufficient durability to make a small size turbocharger commercially viable. Early attempts to use ball bearings were unsuccessful in that sufficient durability could not be achieved. Furthermore, bearing systems for small turbochargers must be capable of mass production manufacturing methods, be low in cost, and easily serviced in the field.
Research and development tests during the 1960's resulted in the perfection of floating sleeve-bearing systems that were capable of suppressing the problems of shaft instability, had acceptable friction losses and achieved satisfactory durability when used on a variety of internal combustion engine turbochargers. Several of these successful bearing systems are illustrated in U.S. Pat. Nos. 3,056,634; 3,096,126; 3,390,926; 3,993,370; and 4,641,977. The bearings of the patents listed above generally solved the stability problem by using a free-floating bushing between the rotating shaft and its stationary supporting member which was adapted to provide a film of lubrication between its inner surface and the rotating shaft and also between its outer surface and the stationary supporting member. In these systems, the free-floating bushings were free to rotate, but at speeds only a fraction of the speed of the rotating shaft and were free to move radially in order to allow the rotating assembly to find and rotate about its center of mass. The inner and outer oil films provided the necessary lubrication to prevent wear and provided a cushion against vibration and shock loads.
In the sleeve bearing systems described above, it was necessary to provide a thrust bearing to sustain the axial loads imposed on the rotating assembly by the actions of the compressor and turbine wheels used in the turbochargers, and a collar was provided on the rotating shaft to bear against a stationary thrust member. However, the high rotational speed of the collar attached to the shaft resulted in a high thrust frictional loss which, in addition to the frictional losses of the sleeve bearings, resulted in a substantial total frictional loss for the complete bearing system.
Since it is advantageous to have a bearing system that has a high mechanical efficiency, the use of anti-friction bearings in high-speed machines such as turbochargers is advisable. U.S. Pat. No. 4,370,106 discloses a bearing system for a turbocharger rotor consisting of an anti-friction ball bearing at its compressor end and a sleeve bearing at its turbine end. In this system, both the anti-friction bearing and the sleeve bearing are mounted in a non-rotating elongated cylinder. The cylinder containing the ball and sleeve bearings is prevented from rotating by a square portion at the compressor end that engages stops in the stationary housing member. Lubricant is provided between the non-rotating cylinder and the supporting housing to provide damping for eccentric motion of the rotor due to residual unbalance. In this bearing system, however, the differential speed between the sleeve bearing and rotor is the very high rotative speed of the rotor. Since sleeve bearing frictional losses are proportional to the square of the differential rotating speed, this system has an inherent higher frictional loss than a full-floating sleeve bearing system. Also, since the non-rotating cylinder that contains the bearings must engage the stationary housing member, it carries the full thrust load of the rotor. The residual imbalance in the rotor forces the non-rotating cylinder to move orbitally, causing the mating surfaces to be subject to fretting. Thus a solid film lubricant must be placed between the mating surfaces to mitigate the fretting problem; however, this problem remains an inherent disadvantage with this type of non-rotating cylinder system and contributes to a limited service life in the field.
The fretting problem inherent with non-rotating systems that are allowed to move radially is solved in the bearing system disclosed in U.S. Pat. No. 4,641,977. In this bearing system, a ball bearing is mounted in an elongated cylinder that has a radially extending flange at one end. The elongated cylinder is free to move radially to a limited degree and free to rotate in the stationary supporting member. The radially extending flange engages the stationary housing to carry the thrust load of the rotor. However, since the elongated cylinder rotates at relatively low speeds, the thrust losses are minimal. In this bearing system, a full-floating sleeve bearing is located at the opposite end of the elongated cylinder to complete the bearing system for carrying the rotor. The frictional losses with this system are reduced due to the ball bearing and floating sleeve bearing, and the mechanical efficiency of the system is relatively high compared to prior bearing systems.
Continued development work has resulted in the systems described in my pending patent application Ser. No. 10/369,801, filed Feb. 20, 2003, which is a continuation-in-part of patent application Ser. No. 09/978,935, filed Oct. 16, 2001. The system described in these patent applications comprises a bearing system with an angular contact ball bearing in each end of a rotatable elongated cylinder that has a radially extending flange at one end and is carried by a stationary bearing housing. Each angular contact ball bearing carries thrust in one direction only, the directions being opposite to one another. The radially extending flange on the end of the rotatable elongated cylinder engages the stationary bearing housing to carry the thrust load of the rotor in both directions. The rotatable elongated cylinder is supplied with a lubricant between its outer diameter and the stationary bearing housing, and this lubricant provides a shock and vibration cushion for the rotating assembly. The rotatable elongated cylinder is provided with passageways that carry the lubricant from its outer surface to the angular contact ball bearings in the ends of the cylinder. A tolerance bearing ring is used between the outer race of the turbine end ball bearing and the stationary bearing housing to allow axial movement of the bearing due to axial expansion of the shaft when heated, while at the same time preventing rotation of the outer race in the housing.
This bearing system has proven to be very satisfactory for commercial use in high-speed turbochargers; however, it requires a supply of pressurized lubricant from the engine lubricating system and, historically, the use of lubricating oil in turbochargers has given rise to a number of operational problems.
To prevent oil leakage into the compressor casing and turbine casings, piston ring seals are employed in commercial turbochargers. Since the piston rings are not positive contact seals, there is a small leak path around the piston rings and, during certain operating conditions of the engine, leakage can occur. Any leakage of lube oil into the turbine casing of the turbocharger can contribute to undesirable emissions in the engine exhaust. Oil leakage into the compressor casing gets carried into the engine intake system and is subsequently burned in the engine cylinder. This also can create undesirable emissions in the engine exhaust.
In addition, in cold weather, there can be a significant lag in providing a satisfactory flow of lubricant to the turbocharger bearings when the engine is initially started. This lag can contribute to failure of the bearings where excessive time is required for the cold viscous lubricant to reach the turbocharger bearings.
Another problem arises when an engine is shut down after being operated at high load where the exhaust gas temperatures are very high. Heat can be transferred into the turbocharger bearing housing from the hot exhaust manifold, and residual oil in the turbocharger bearing housing can carbonize. This carbonization build-up can eventually lead to failure of the bearing system.
Finally, there is the cost of the mechanical features involved in piping lube oil from the engine to the turbocharger oil inlet and piping the expended lube oil to the engine crankcase.